Folding drafting units are used in order to draw fibres of a sliver into a fine ribbon before spinning. This drawing of the sliver is also designated as drafting. Drafting units used at the present time are, for example, four-roller drafting units which have two roller groups standing perpendicularly to one another. A third pair of rollers carries aprons which fold the fibre material emerging from the second pair of rollers and deliver it in this state to a fourth pair of rollers, by which the fibre material is drafted to final fineness. Such a drafting unit is known, for example, from WO-A 2013/041220.
In folding drafting units used at the present time, the sliver can acquire a draft of 150 to 400 times. The disadvantage, however, is that, in the case of very high drafts, the unevenness of the fibre material emerging from the drafting unit increases, and this may lead to unacceptable yarn unevenness. In contrast to this, in the case of low drafts upstream of the folding zone, compacting by folding is not optimal, and this may cause the fibre material to expand in the nip region of the last pair of rollers, consequently impeding the subsequent spinning process.
The object of the present invention was, therefore, to provide a folding drafting unit, by means of which, even in the case of high drafts, maximum evenness and a minimum exit width of the emerging fibre material can be achieved.